1. Field of the Invention
This invention relates to the vapor condensation heating of articles to an elevated temperature and, more particularly, to a method and apparatus for generating a hot saturated vapor body that is controllably exposed to only a selected surface of an article to be heated, while continuously minimizing any loss of the vapor to the atmosphere.
2. Description of the Prior Art
In performing a solder, fusing or brasing operation on an article, for example, it is necessary to heat the article to a predetermined elevated temperature sufficient to perform the desired operation thereon. While the present invention is not to be construed as limited to a particular type of heat-induced operation, the nature thereof is most readily understood in the context of performing a soldering (or unsoldering) operation on an article, particularly on one surface thereof, such as on a selected underside surface of a printed circuit board during the manufacture or repair thereof.
In that connection, it is realized that whenever two or more elements, one of which may comprise a component lead or terminal, and the other a metallized pad or land area of a circuit board, are to be solder-connected, such elements must be heated to an elevated temperature sufficient to melt an interposed solder coat (or preform), while simultaneously preferably protecting the elements to be joined from oxidation during the soldering operation. In a typical hand soldering operation, utilizing a soldering iron, as well as in a conventional wave soldering operation, a coating of flux has normally been required, and applied on at least the article areas to be soldered, in order to minimize any deleterious oxide surface build-up during soldering.
As a result of a need to perform mass (or selective) soldering or unsoldering operations on articles, such as on complex printed circuit boards, which, for example may required hundreds (or even thousands) of closely spaced connections to be soldered, while at the same time obviating the problem of surface oxidation in the absence of flux, there has recently been increasing use made of vapor condensation soldering processes and apparatus. One such process and apparatus of that type is the subject of U.S. Pat. No. 3,866,307 of R. C. Pfahl, Jr. et al., issued Feb. 8, 1975, assigned to the assignee of the present invention, and incorporated herein by reference.
In accordance with the teaching in the prior Pfahl et al. patent, the article to be soldered (fused or brazed) is placed within a vessel that is open to the atmosphere on the top side so as to facilitate the entry and removal of the article therefrom. Each article to be heated to a desired elevated temperature is immersed within a primary body of hot saturated vapor generated within the vessel, with a portion of the vapor body condensing on the article and transferring thereto its latent heat of vaporization. This heats the article to the temperature required to perform a soldering operation, for example, thereon. The hot saturated vapor body is generated by continuously boiling within the vessel a heat transfer liquid that is non-conducting, chemically inert, and has a boiling point at least equal to, but preferably above, the temperature required to melt the solder. Such a vapor condensation facility may also be employed to perform a mass re-flow soldering operation on a continuously moving line of articles.
The various preferred heat transfer liquids presently employed to heat articles in the manner described above, and which liquids are described in greater detail hereinbelow, are quite expensive. As such, any appreciable loss of the generated vapor in question to the atmosphere significantly impacts on the over-all costs incurred in carrying out a given soldering operation, particularly high volume, mass soldering operations.
One technique utilized heretofore to at least partially minimize the loss of the relatively expensive primary vapor to the atmosphere in an open top vessel has involved positioning a suitable cooling coil (or coils) about the inner sidewalls of the vessel at an elevation near the top thereof. Such a cooling coil (or coils) condenses any vapor that rises to the elevation of, and in the immediate vicinity of, the coils. This technique, however is not completely effective in condensing the major portion of the rising vapor in the central region of the vessel. A vessel that incorporates both a peripherially disposed cooling coil and a completely enclosing, but removable, top wall or cover is disclosed in U.S. Pat. No. 4,022,371 of E. R. Skarvinko et al. Such an apparatus, of course, not only requires the total immersion of the articles within the vapor, but the removal and re-positioning of the cover plate from the vessel in connection with each heating operation, with the attendant loss of vapor to the atmosphere at such times.
A more effective technique recently developed to minimize the loss of the relatively expensive heat transfer liquid to the atmosphere, while in vapor form in an open top vessel, is the subject of U.S. Pat. No. 3,904,102 of T. Y. Chu et al., issued Sept. 9, 1975, also assigned to the assignee of the present invention. In accordance with the technique disclosed in the last-mentioned reference, a secondary body of vapor, generated by boiling a relatively inexpensive heat transfer liquid, is interposed between the relatively expensive primary body of vapor and the atmosphere. This technique substantially reduces loss to the atmosphere of the hot primary body of vapor confined therebelow.
Although such a secondary body of vapor has been found to be quite effective in reducing the losses of the expensive primary vapor, portions of both the primary and secondary vapors are nevertheless still lost to the atmosphere across the secondary vapor-air interface. One reason for this is believed to be the disturbance produced at the primary-secondary vapor interface when normally generating the secondary vapor. The dual vapor losses in question are at least substantially further minimized, however, in accordance with a method and apparatus for more effectively maintaining the secondary vapor body, disclosed in U.S. Pat. No. 4,055,217 of T. Y. Chu et al., issued Feb. 2, 1976, also assigned to the assignee of the present invention.
With respect to all of the aforementioned dual vapor body generating condensation systems, it is appreciated, of course, that the articles to be heated must be passed downwardly through the upper secondary vapor body in order to be immersed in the primary vapor body. This presents no serious problem with respect to many articles, including certain types of printed circuit boards with only printed circuitry thereon, or having components and/or devices mounted thereon which are not adversely affected by the elevated temperatures of the primary vapor body, in particular.
In an ever-increasing number of mass soldering circuit board applications today, however, the mounted active and passive electronic devices and/or components, particularly when of the solid state integrated circuit type, cannot be safely subjected to a hot saturated vapor body for even relatively short periods of time, and especially at the elevated temperatures required for soldering. In such cases, and with particular reference to circuit boards, with components mounted on only one side, it would be very desirous to controllably expose only the non-component, printed circuit side thereof to be soldered (hereinafter referred to simply as the underside) to a single hot, saturated (primary) vapor body confined within a vessel, i.e., with no immersion of the completely assembled circuit board within the vapor body.
Such a technique would also be of considerable advantage in the repair of circuit boards, wherein both unsoldering and resoldering operations are normally involved. In this regard, it would likewise be very beneficial if only selected discrete areas on the underside of the circuit board would have to be subjected to the heat of vaporization of a generated body of vapor while, at the same time, minimizing the loss of any vapor to the atmosphere in the absence of any overlying secondary vapor blanket.
One technique employed heretofore to heat only the underside of a printed circuit board in a vapor condensation apparatus has involved a vessel which incorporated a horizontally disposed, and retractable planar cover plate. The latter was dimensioned and adapted to allow the selective closure of a central opening formed in the top wall of the vessel. Associated mounting structure secured to the top wall of the vessel allowed a circuit board to be supported thereon at an elevation immediately above the cover plate in such a manner that either the latter, or the circuit board, could be selectively employed to effectively close the upper top wall vessel opening, so as to prevent loss of vapor to the atmosphere therethrough.
While the latter apparatus was of simplified and inexpensive construction, and allowed controllable heating of only the underside of a circuit board (or any similar type substrate), it unfortunately was found to still allow an appreciable amount of the hot body of vapor generated within the vessel to be periodically lost to the atmosphere. This would happen each time a small amount of the vapor would become entrapped between the underside of each successively mounted circuit board and the adjacent upper surface of the cover plate. This could occur, of course, as a result of the following sequence of operations: (1) mounting a circuit board on the top side of the vessel, so as to overlie the opening therein, while the retractable cover plate is also in a fully extended vessel-closing, underlying position; (2) retracting the cover plate to expose the underside of the circuit board to the hot vapor body therebelow; (3) advancing the cover plate so as to agin close the top side of the vessel, while simultaneously isolating the major (as distinguished from minor) portion of the vapor body therebelow, and finally, (4) removing the soldered (or unsoldered) circuit board from the vessel, which allows the minor (entrapped) portion of the vapor body to escape to the atmosphere.
As the cover plate in the last mentioned prior apparatus did not incorporate any means for cooling the major surfaces thereof, it could not effectively condense my of the relatively expensive entrapped vapor that would ultimately be lost to the atmosphere. With the preferred types of heat transfer liquid, as previously noted, being relatively expensive, the resulting cumulative vapor losses in apparatus of the type in question could very readily adversely affect the total costs incurred, for example, in any high volume mass soldering operation.
An article entitled "Solvent Vapor Solder Reflow", by E. G. Dingman, IBM Technical Disclosure Bulletin, Vol. 13, No. 3, dated August 1970, describes the use of a boiling solvent (such as that sold under the tradename "Freon E5", by E. I. DuPont de Nemours and Company) to facilitate the removal and resolder of electronic components during printed circuit board rework operations. It is stated therein that "The solvent condenses only on the areas having a temperature lower than the boiling point of the solvent used. This releases the heat of vaporization and enables solder rework operations with materials and components that are heat sensitive. The rapid and selective application of heat to small areas with high thermal conductivity is possible within a matrix of material which is heat sensitive and cannot tolerate high temperatures." While this disclosure discusses the rapid and selective application of heat to small areas of high thermal conductivity, such as the metallic pads, land areas, lead ends and circuit paths, of printed circuit boards, there is no suggestion of how to controllably expose a hot saturated vapor body either to only one surface of a printed circuit board having both low and high thermal conductivity areas thereon or, alternatively, to only selective discrete regions encompassed within the areas of high thermal conductivity. Moreover, no physical structure is either illustrated, or described, for accomplishing even the described mode of operation and, particularly, in relation to simultaneously preventing or minimizing loss of vapor to the atmosphere.
It was further appreciated heretofore that prior art apparatus existed of the type that required the confinement of an article within a vapor generating vessel incorpoing some form of a fixed and cooled top wall or cover. For example, B. Juettner U.S. Pat. No. 2,716,348 discloses a vessel with a horizontally disposed, water-cooled, top-enclosing cover plate (removable but not retractable). K. A. Holm et al. U.S. Pat. No. 3,479,252 discloses a vessel with water-cooled sidewalls and an air-flow, channel-defining stationary top wall which is formed with a permanent article-receiving opening in a central region thereof.
From the foregoing, it is thus seen that none of the discussed prior art discloses selective means for closing an otherwise open top of a vapor condensation vessel in such a manner that only the underside surface of an article (or a patterned area thereof) may be controllably exposed to, and heated by, a body of hot saturated vapor generated therewithin, while simultaneously minimizing any loss of the vapor to the atmosphere at all times.